Birefringence is a phenomenon that light beam, when irradiating upon an anisotropic crystal, splits into two rays that refract in different paths. Crystals that exhibit birefringent phenomenon are called birefringent crystals. Birefringent crystals can be classified into uniaxial crystal and biaxial crystal. Crystals with trigonal, hexagonal and tetragonal systems are uniaxial crystals; whereas crystals with monoclinic, orthorhombic and triclinic systems are biaxial crystals. The birefringent property is an important optical parameter for electro-optical functional material. In recent years, with the fast development of the optical communication technology, birefringent crystals become key materials for polarization splitting prism such as Glan prism and Wollaston prism, and optical communication devices such as opto-isolator, beam shifter and circulator.
Commonly used birefringent crystal materials include YVO4, rutile, LiNbO3, CaCO3, MgF2, and α-BaB2O4 crystals, etc. YVO4 is a kind of artificial birefringent crystal with good properties; and is easy to grow into large crystals with high optical quality by Czochralski method. However, its optical transmission range is 400-5000 nm and thus cannot be used in the ultraviolet range. Rutile crystal, although possessing a high birefringence, is difficult to be used for manufacturing devices due to its hardness. LiNbO3 crystal can be easily obtained in large sizes, but its birefringence is too low. Calcite is a natural crystal with relatively high content of impurities. The synthesis of calcite is difficult and the crystals are always too small to meet the requirements of large-sized optical polarizers. Regular calcite can be used only in the wavelength range above 350 nm; whereas the calcite of ultraviolet optical grade is not only hard to obtain but also incapable of being used in the deep-ultraviolet range (<200 nm). MgF2 crystal is a good material that can be used in the deep-ultraviolet range. It has a large transmission range (110-8500 nm) but a low birefringence, therefore MgF2 crystal cannot be used for manufacturing Glan prism, but Rochon prism. Nevertheless, its beam separation angle is small and the device is in a large size, which is inconvenient for use. α-BaB2O4, for having solid-state phase transition, is apt to crack in the process of crystal growth. Therefore, it is urgent to explore new birefringent crystal materials that are able to overcome the shortcomings of the current birefringent crystals and satisfy the active demand from the optical communication technology development.
Borates containing alkali or alkaline-earth metal cations possess large optical transmission range and excellent ultraviolet transmission ability, therefore they have important practical value in the optical field especially in the deep-ultraviolet range. The lithium metaborate crystal provided in this invention has the chemical formula of LiBO2. It is a centrosymmetric compound and belongs to monoclinic system. The synthesis and detailed structure of this crystal was first reported in 1964 by W. H. Zachariasen, et al, referring to the reference, Acta Cryst., 17(1964), 749-751. Though there have been some studies about this crystal, the growth of single-crystals in high qualities and large sizes were not reported, neither was their utilization as optical crystals. Our current theoretical studies show that if all of the anion groups in the crystal structure are BO3 and arranged in a parallel or nearly parallel pattern, the birefringence of such a material will tend to be increased. In the structure of the lithium metaborate provided in this invention, the BO3 groups connect each other by sharing corner atoms forming chains in a direction along the b axis, which is beneficial to generate high birefringence. Experimental results have verified that said crystal has very high birefringence, large transmission range and deep-ultraviolet transmission ability, therefore, it is a promising optical material. The method provided in this invention is easy to operate and the crystal grown thereby has good qualities and can be easily used in the fields of birefringent crystal and optical devices, etc.